A magnetic resonance imaging (MRI) apparatus can obtain images representing physical and chemical properties of a subject by utilizing a nuclear magnetic resonance phenomenon which occurs when the subject (object to be examined) placed in a homogeneous static magnetic field space is irradiated with a radio-frequency pulse, and is used particularly for medical purposes. The magnetic resonance imaging apparatus mainly includes a magnet device for generating a homogeneous static magnetic field in an imaging region (a magnetic field space) in which a subject is carried, a radio frequency (RF) coil which irradiates the imaging region with a radio-frequency pulse, a receiving coil which receives a response from the imaging region, and a gradient magnetic field coil which generates a gradient magnetic field for providing position information on a resonance phenomenon into the imaging region.
In the magnetic resonance imaging apparatus, one of requirements for improving the image quality is to improve static magnetic field homogeneity in the imaging region. Therefore, the magnet device for generating a static magnetic field is subjected to magnetic field homogeneity adjustments in steps of design, fabrication (assembly) and installation. Among these adjustments, the magnetic field homogeneity adjustment in the installation step is performed for example by adjusting a magnetic field inhomogeneity component caused by a fabrication error, ambient environment or the like by adding or removing a magnetic material (magnetic material shim) to or from the magnet device. Matters as to at which position and what amount of the magnetic material shims are arranged can be generalized as optimization matters having magnetic field homogeneity in the imaging space as an objective function. In other words, the arrangement of magnetic material shims can be determined by a linear optimization method or its improved approach or the like by using a given magnetic field distribution in the imaging space.
Patent Literature 1 discloses a method in which, in order to improve work efficiency for the arrangement of magnetic material shims determined by these methods, for each position at which a volume distribution of magnetic material shims to be arranged has a local maximum value or local minimum value, magnetic material shims having volumes obtained by multiplying volume distributions in a region around the position are arranged, and thereby, the number of magnetic material shims to be arranged is significantly reduced.